Build It! A Portable Headphone Amplifier
By Mark Amundson
Ever wanted to monitor a line level signal? Or want to provide another
headphone output to share the multi-track mix? Well, this Do-It-Yourself
(DIY) project is for you. With the modern Musical Instrument (MI) grade
of equipment, many situations arise where the need to individually monitor
mixer channel inputs for adjustment of signal levels is not available. Also
there are times when a portable signal sniffer is just the ticket to
troubleshoot sound system interconnects.
A Little Design Philosophy
I chose this project to teach a little circuit design as well as arrive with a
headphone amplifier. One could have grabbed a couple of LM386 Audio
Amplifier Integrated Circuits (ICs), added a few passive parts, and
completed the project. But I chose a discrete version instead. The rationale
to avoid the purpose built ICs is thin but does have its merits. One is to
break out the heat generating output transistors to avoid the thermal
changes from affecting the other transistors. Another is to prove to our
younger generation that before integrated circuits, simple good sounding
audio amplifiers were possible in small sizes.
This project also has the goal of using commonly available parts so that
first time DIY project builders would feel comfortable finding the parts and
use assembly techniques like perferated board or etch your own circuit
board. The transistors are garden variety 2N2222 and 2N2907 types and
either plastic or metal case types can be used. The diodes are 1N4001
rectifier and 1N4148 switching types that can be easily found. All the
resistors are 5% carbon film, quarter watt values and the capacitors should
16 volt or higher voltage ratings.
Circuit Description
This portable headphone amplifier project consists of two audio amplifier
circuits for stereo operation powered by a switched 9 volt alkaline battery.
Each audio amplifier circuit contains a 100 kilo-ohm, audio taper
potentiometer on a common shaft for volume control. A common stereo
headphone jack (ring-tip-sleeve) is used along with four phone jacks (tipsleeve) for input signals. Stereo/mono switching is easily possible as well.
Figure 1 shows the top level schematic.
Each audio amplifier circuit consists of six transistors, three diodes, nine
capacitors, and fourteen resistors. Figure 2 shows the circuit schematic.
This circuit is best described in backwards fashion; from output towards
the input. Transistors Q1 and Q4 form the push-pull output stage and are
configured for rail to rail operation. This permits the output voltage swing
to approach the battery supply voltage for best peak-to-peak capability.
Diode D3 is placed in series with the battery’s positive lead to prevent
accidental reverse battery connections from destroying the circuit.
Capacitor C2 is a 470 microfarad electrolytic device for decoupling the DC
offset voltage from the push-pull output to the ground referenced
headphone load.
Resistor R3 and Capacitor C1 form a high frequency load to keep the
amplifier stable in the presence of inductive loading of the headphone
voice coils. Resistors R15 and R16 are 4.7 ohm resistors that cover a
couple functions. Their prime purpose is to provide some degeneration of
drive signals on heavier headphone loads. Most professional studio
headphones and personal “Walkman” stereo headphones have voice coil
impedances of 30 ohms or higher. However, classic home stereo
headphones have an impedance of 8 ohms or more. These lower
impedances draw more current from the amplifier. The 4.7 ohm resistors
drop more voltage and limit the output dissipation to about 350 milliwatts.
These resistors also prevent accidental short circuits from destroying the
output transistors.
Transistors Q2 and Q3 plus resistors R1 and R2 form the driver stage of
the audio amplifier circuit. By sharing the 4.7 ohm resistor’s voltage drop
on the driver transistor’s emitters, the above mentioned drive degeneration
occurs. Resistors R1 and R2 are not strictly necessary but are added to
achieve better fidelity of sound by helping drain capacitive charges
accumulated on the base-emitter junctions of the output transistors.
Resistor R5 through R8, along with diodes D1 and D2, capacitors C3 and
C8, and Transistor Q5 form the pre-driver stage of the audio amplifier
circuit. One kilo-ohm resistors R6 and R7 with capacitor C6 tapped in
between, form a collector load for transistor Q5 plus a “poor man’s” or
bootstrap constant current source for the driver stage biasing. By selecting
C3 at 100 microfarads, the capacitor attempts to retain a constant voltage
across R7 which forms the current source. The reason to desire a constant
current source is that diodes D1 and D2 plus R5 will maintain a constant
voltage drop for optimum driver transistor biasing. Resistor R8 keeps
transistor Q5’s gain under control via emitter degenerative feedback.
Capacitor C8 forms a miller capacitance to minimize circuit oscillation and
keep the amplifier stable.
The balance of the components in the audio amplifier circuit make up the
amplifier’s input stage. Resistors R4, R12, and R13 together with
capacitors C5 and C9 provide stable bias voltage for input transistor Q6 to
ensure that the whole amplifier circuit centers up about one-half of the
supply voltage. Resistor R11 is sized at one kilo-ohm for almost a one volt
(one milliampere) direct coupled bias for transistor Q5. Resistor R9
performs double duty as an emitter resistor and uses the 470 ohms in
conjuction with the 22 ohms of resistor R10 as the gain setting part of the
amplifier. Capacitors C4 and C7 set the low and high frequency response
of the circuit respectively. Potentiometer R14 and capacitor C6 provide
signal input to the base of transistor Q6 and provide decoupling for the
incoming signal.
Project Construction
The portable headphone amplifier project construction can be done in
many ways. As mentioned earlier, board type construction is the preferred
method of circuit interconnection. Figures 3 and 4 show a prospective
copper clad circuit board layout. Other proto-board types can also be used.
If you try to layout your own board, use the schematic to help physically
place your components. Back in my school day’s, my instructor would
teach us to paste a paper schematic to a wood board, pound small brass
nails into the circuit node points, and solder wire or components across the
nail heads for interconnections. The term “Breadboarding” came from this
early form of project construction since a kitchen’s breadboard was an
ideal flat piece of wood for usage. Figure 5 shows an inside view of the
author’s internal construction.
Project encasement can be done using a number of choices. Aluminum
project boxes are my favorite by lending ruggedness in construction and
providing an easy surface for drilling and painting. As with other projects,
consider what conveniences your local hardware store offers for securing
parts or dressing up your project. Adhesive backed foam weather stripping
is one of my favorite items for securing batteries and circuit boards in
project boxes; due to its insulating and light compression properties. In a
rough and tumble world of gigs, rattles and loose hardware spell danger.
Project wiring can be done using non-critical types of insulated conductors.
My favorite is PVC insulated solid conductor colored wires of small gauges
for the stiffness of bending and for a clean lead dress in the project box.
Assorted colors of wire wrap (Kynar) wire are also handy for interconnects.
Shielded conductors for the input signal routing can be avoided if you use a
conductive project case for shielding instead.
Circuit Performance
The next step after project construction is the dreaded “smoke test”. Like
the old, wise electrical technician said, “Every component has smoke
inside it. You pass the smoke test if you do not let the smoke out.” If
possible use a DC ammeter between each amplifier circuit and the battery.
Each amplifier circuit should draw close 8 to 10 milliamperes of battery
current with no signal input. The junction of the 4.7 ohm resistors (or
board jumper) should have about a 4 to 5 volt reading to ground using a
DC voltmeter. If trouble is present, trace through your circuit connections,
using a copy of the schematic and a hi-lighter pen to verify the
construction.
Project Usage
The portable headphone amplifier as shown contains four phone jacks in
which they are paired (bridged) to permit tapping into lines for monitoring
a signal path. If you desire to troubleshoot signals or just use mixer insert
jacks, then only a single jack per stereo side will be needed. With the
optional mono/stereo switch, a single line connection can be made. Figure
6 shows a typical application.
ABOUT THE AUTHOR
Mark Amundson’s interest in music and electronics started in 1972 and
1974, respectively. He is an Analog and RF Development Engineer and has
been building and modifying audio projects for years, since his first onetube radio kit. His favorite instruments are guitars, electric and acoustic,
and keyboards. Mark plays regularly with a local band, and has spent over
two decades as a musician and soundman.
Table 1
PARTS LIST
REF. DES.
PART
BATTERY
C1
C2, C9
C3
C4
C5
C6
C7
C8
D1, D2
J1
J2, J3, J4
J5
Q1, Q3, Q6
Q2, Q4, Q5
R1, R2, R5
R3
R4
R6, R7, R11
R8
R9
R10
R12
R13
R14
S1
MISC
9volt battery with battery clip and/or holder
.1uf, 50volt or more ceramic capacitor
470uf, 16volt or more electrolytic capacitors
100uf, 16volt or more electrolytic capacitor
220uf, 16volt or more electrolytic capacitor
10uf, 16volt or more electrolytic capacitor
1uf, 16volt or more electrolytic capacitor
100pf, 50volt or more ceramic capacitor
470pf, 50volt or more ceramic capacitor
1N4148 or 1N914 Switching diodes
3 conductor open circuit phone jack
2 conductor open circuit phone jack
2 conductor closed circuit phone jack
2N2907 or PN2907 PNP transistors
2N2222 or PN2222 NPN transistors
100 ohm, ¼ watt resistors
10 ohm, ¼ watt resistor
47k ohm, ¼ watt resistor
1k ohm, ¼ watt resistors
47 ohm, ¼ watt resistor
470 ohm, ¼ watt resistor
22 ohm, ¼ watt resistor
10k ohm, ¼ watt resistor
5.6k ohm, ¼ watt resistor
dual 100k ohm, audio taper potentiometer
SPST switch
Case, Circuit board, Wire, Knob